Dynamic contrast enhanced magnetic resonance imaging of bladder cancer and implications for biological image-adapted radiotherapy

Pharmacokinetic analysis and k-means clustering of DCEMR images for radiotherapy outcome prediction of advanced cervical cancers

INTRODUCTION

Pharmacokinetic analysis of dynamic contrast enhanced magnetic resonance images (DCEMRI) allows for quantitative characterization of vascular properties of tumors. The aim of this study is twofold, first to determine if tumor regions with similar vascularization could be labeled by clustering methods, second to determine if the identified regions can be associated with local cancer relapse.

MATERIALS AND METHODS

Eighty-one patients with locally advanced cervical cancer treated with chemoradiotherapy underwent DCEMRI with Gd-DTPA prior to external beam radiotherapy. The median follow-up time after treatment was four years, in which nine patients had primary tumor relapse. By fitting a pharmacokinetic two-compartment model function to the temporal contrast enhancement in the tumor, two pharmacokinetic parameters, K(trans) and ύ(e), were estimated voxel by voxel from the DCEMR-images. Intratumoral regions with similar vascularization were identified by k-means clustering of the two pharmacokinetic parameter estimates over all patients. The volume fraction of each cluster was used to evaluate the prognostic value of the clusters.

RESULTS

Three clusters provided a sufficient reduction of the cluster variance to label different vascular properties within the tumors. The corresponding median volume fraction of each cluster was 38%, 46% and 10%. The second cluster was significantly associated with primary tumor control in a log-rank survival test (p-value: 0.042), showing a decreased risk of treatment failure for patients with high volume fraction of voxels.

CONCLUSIONS

Intratumoral regions showing similar vascular properties could successfully be labeled in three distinct clusters and the volume fraction of one cluster region was associated with primary tumor control.

Non-invasive imaging of combretastatin activity in two tumor models: Association with invasive estimates

INTRODUCTION

The efficacy of the vascular disrupting agent combretastatin A-4 phosphate (CA4P) depends on several factors including tumor size, nitric oxide level, interstitial fluid pressure, and vascular permeability. These factors vary among tumor types. The aim of this study was to investigate all these factors in two tumor models that respond differently to CA4P.

MATERIAL AND METHODS

Mice bearing C3H mammary carcinomas or KHT sarcomas (200 to 800 mm(3)) were intraperitoneally injected with CA4P (100 mg/kg). Tumor size and the effect of a nitric oxide inhibitor nitro-L-arginine (NLA) administered intravenously were evaluated by necrotic fraction histologically assessed at 24 hours. Interstitial fluid pressure (IFP) was measured using the wick-in-needle technique, and vascular characteristics were assessed with dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI).

RESULTS

Initial necrotic fraction was about 10% in both tumor models at 200 mm(3), but only increased significantly with tumor size in the C3H mammary carcinoma. In this tumor, CA4P significantly induced further necrosis by about 15% at all sizes, but in the KHT tumor, the induced necrotic fraction depended on tumor size. For both tumor types, NLA with CA4P significantly increased necrotic fraction above that for each drug alone. CA4P significantly decreased IFP in all tumors except in the 800 mm(3) C3H tumor, which had an initially non-significant lower value. Interstitial volume estimated by DCE-MRI increased in all groups, except the 800 mm(3) C3H tumors. DCE-MRI vascular parameters showed different initial characteristics and general significant reductions following CA4P treatment.

CONCLUSIONS

Both tumor models showed differences in all factors before treatment, and in their response to CA4P. Perfusion and permeability as estimated by DCE-MRI play a central role in the CA4P response, and interstitial volume and IFP seemed related. These factors may be of clinical value in the planning of CA4P treatments.

Preclinical dynamic 18F-FDG PET - tumor characterization and radiotherapy response assessment by kinetic compartment analysis

BACKGROUND

Non-invasive visualization of tumor biological and molecular processes of importance to diagnosis and treatment response is likely to be critical in individualized cancer therapy. Since conventional static (18)F-FDG PET with calculation of the semi-quantitative parameter standardized uptake value (SUV) may be subject to many sources of variability, we here present an approach of quantifying the (18)F-FDG uptake by analytic two-tissue compartment modeling, extracting kinetic tumor parameters from dynamic (18)F-FDG PET. Further, we evaluate the potential of such parameters in radiotherapy response assessment.

MATERIAL AND METHODS

Male, athymic mice with prostate carcinoma xenografts were subjected to dynamic PET either untreated (n=8) or 24 h post-irradiation (7.5 Gy single dose, n=8). After 10 h of fasting, intravenous bolus injections of 10-15 MBq (18)F-FDG were administered and a 1 h dynamic PET scan was performed. 4D emission data were reconstructed using OSEM-MAP, before remote post-processing. Individual arterial input functions were extracted from the image series. Subsequently, tumor (18)F-FDG uptake was fitted voxel-by-voxel to a compartment model, producing kinetic parameter maps.

RESULTS

The kinetic model separated the (18)F-FDG uptake into free and bound tracer and quantified three parameters; forward tracer diffusion (k(1)), backward tracer diffusion (k(2)), and rate of (18)F-FDG phosphorylation, i.e. the glucose metabolism (k(3)). The fitted kinetic model gave a goodness of fit (r(2)) to the observed data ranging from 0.91 to 0.99, and produced parametrical images of all tumors included in the study. Untreated tumors showed homogeneous intra-group median values of all three parameters (k(1), k(2) and k(3)), whereas the parameters significantly increased in the tumors irradiated 24 h prior to (18)F-FDG PET.

CONCLUSIONS

This study demonstrates the feasibility of a two-tissue compartment kinetic analysis of dynamic (18)F-FDG PET images. If validated, extracted parametrical maps might contribute to tumor biological characterization and radiotherapy response assessment.

Influence of MLC leaf width on biologically adapted IMRT plans

INTRODUCTION

High resolution beam delivery may be required for optimal biology-guided adaptive therapy. In this work, we have studied the influence of multi leaf collimator (MLC) leaf widths on the treatment outcome following adapted IMRT of a hypoxic tumour.

MATERIAL AND METHODS

Dynamic contrast enhanced MR images of a dog with a spontaneous tumour in the nasal region were used to create a tentative hypoxia map following a previously published procedure. The hypoxia map was used as a basis for generating compartmental gross tumour volumes, which were utilised as planning structures in biologically adapted IMRT. Three different MLCs were employed in inverse treatment planning, with leaf widths of 2.5 mm, 5 mm and 10 mm. The number of treatment beams and the degree of step-and-shoot beam modulation were varied. By optimising the tumour control probability (TCP) function, optimal compartmental doses were derived and used as target doses in the inverse planning. Resulting IMRT dose distributions and dose volume histograms (DVHs) were exported and analysed, giving estimates of TCP and compartmental equivalent uniform doses (EUDs). The impact of patient setup accuracy was simulated.

RESULTS

The MLC with the smallest leaf width (2.5 mm) consistently gave the highest TCPs and compartmental EUDs, assuming no setup error. The difference between this MLC and the 5 mm MLC was rather small, while the MLC with 10 mm leaf width gave considerably lower TCPs. When including random and systematic setup errors, errors larger than 5 mm gave only small differences between the MLC types. For setup errors larger than 7 mm no differences were found between non-uniform and uniform dose distributions.

CONCLUSIONS

Biologically adapted radiotherapy may require MLCs with leaf widths smaller than 10 mm. However, for a high probability of cure it is crucial that accurate patient setup is ensured.